The present disclosure relates generally to a power transmission device operable to selectively transfer torque between first and second sets of drivable wheels of a vehicle. More particularly, the present disclosure is directed to a power transmission device with a hydrostatic torque control system.
Due to increased demand for four-wheel drive vehicles, power transmission systems are more frequently being incorporated into vehicle driveline applications for transferring drive torque to the wheels. Some vehicles include a power transmission device operably installed between the primary and secondary drivelines. Such power transmission devices are typically equipped with a torque transfer mechanism for selectively transferring drive torque from the primary driveline to the secondary driveline to establish a four-wheel drive mode of operation. At least one known torque transfer mechanism includes a dog-type lock-up clutch that may be selectively engaged for rigidly coupling the secondary driveline to the primary driveline when the vehicle is operated in the four-wheel drive mode. When the lock-up clutch is released, drive torque is delivered only to the primary driveline and the vehicle operates in a two-wheel drive mode.
Another type of power transmission device, referred to as a transfer case, may be operable to automatically direct drive torque to the secondary wheels without any input or action on the part of a vehicle operator. When traction is lost at the primary wheels, a four-wheel drive mode is entered. Some transfer cases are equipped with an electrically-controlled clutch actuator operable to regulate the amount of drive torque transferred through a friction clutch to a secondary output shaft. The actuator typically includes an electric motor to provide an application force to the friction clutch.
While many power transfer devices are currently used in four-wheel drive vehicles, a need exists to advance the technology. For example, packaging concerns, weight and electrical power requirements of the power transmission device may make such systems cost prohibitive in some four-wheel drive applications.
A power transmission device with a ring gear and a carrier assembly that are coupled to first and second rotary members, respectively. The carrier assembly includes pump assemblies, each of which having a rotor pin, an inner rotor rotatably received on the rotor pin, and an outer rotor that is rotatably received over the inner rotor. The outer rotors have planet gear teeth that are meshingly engaged with teeth of the ring gear. First and second fluid conduits are coupled to first and second sides of the pump assemblies, respectively, to facilitate the flow of hydraulic fluid into and out of the pump assemblies when they are operated in a first rotational direction. A valve is coupled to at least one of the first and second fluid conduits. The valve has a valve element, which is movable between first and second positions, for controlling fluid flow through the valve.